I love learning about nature!
I looked up about what happens to the mass you lose when you lose weight. A paper last month in the peer-reviewed BMJ (formerly British Medical Journal), “When somebody loses weight, where does the fat go?” confirmed what I had read casually: you exhale it, or rather you exhale 84% of it. The rest is water, which we excrete “in the urine, faeces, sweat, breath, tears, or other bodily fluids.”
The paper’s lead author, a physicist, also wrote simplified versions of the chemical reactions. The paper is mostly readable to a lay person, such as me, and is short so I recommend reviewing it. It also begins with a graph showing that nearly no one the authors asked knew the answer or even answered that they didn’t know.
An article interviewing the author, “When you lose weight, where does the fat go? Most of the mass is breathed out as carbon dioxide, study shows,” in a site called Science Daily gives some background on his motivation and process of finding out.
Though that article came out last month, a post from the Mayo Clinic from two years ago, “When you lose weight, where does the lost body fat go?“, briefly wrote that the mass gets “excreted in your urine and sweat or exhaled from your lungs,” though it didn’t describe the metabolic processes or calculate the percentages.
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